An optical fiber switch includes an optical fiber with a birefringence which can maintain an input signal in one of two perpendicular polarization paths and a laser pump coupled to the optical fiber and which can generate a gating signal. The optical fiber switch operates by having an input signal propagate in one of the two perpendicular polarization modes in the optical fiber until a gating signal is input into the optical fiber in the same polarization mode as the input signal is propagating in. If the gating signal has sufficient power, the gating signal induces non-linear birefringence in the optical fiber causing the input signal to switch orientation and propagate in the other polarization mode. To maintain polarization, the optical fiber in the optical fiber switch described above is selected to be strongly birefringent, i.e. 10.sup.-4 or greater, and if the optical fiber is strongly birefringent, then the optical fiber has a small effective area, i.e. less than 40 microns.sup.2.
The main problem with the optical fiber switch described above is that an input signal in the optical fiber switch experiences an undesirable amount of non-linear effects which degrade the input signal. The nonlinear effects include fiber fuse, which is discussed in "Experimental Investigation of the Fiber Fuse," by D. D. Davis & S. C. Mettler, in Optical Fiber Conference, WP17, pp. 186-187, 1995, which is herein incorporated by reference, the creation of higher order solitons due to Raman effects, and cross-phase modulation which is explained in greater detail below. The non-linear effects occur because of the high birefringence of the optical fiber, the small effective area of the optical fiber, and the high power level required for the gating signal. In particular, the input signal experiences an undesirable amount of nonlinear effects because of the high power level required for the gating signal to switch the input signal. The high power level for gating signal is needed because of the high birefringence of the optical fiber.
As discussed above, one of the non-linear effects experienced by the input signal and which increases as the power level of the gating signal increases is cross-phase modulation. The amount of cross-phase modulation experienced by the input signal can be estimated by the following equation : EQU .DELTA..phi.=(2.pi./.lambda.)*[L*N.sub.2 *I]
where L is the length of the optical fiber, N.sub.2 is the index of refraction of the core, and I is the power level or intensity of the gating signal in watts. As can be seen from this equation, the amount of cross-phase modulation experienced by the input signal increases as the power level or intensity of the gating signal increases. Accordingly, reducing the power level of the gating signal will reduce the amount of cross-phase modulation.